Conversion of hydrocarbons



compared with the original charge.

4 Patented July 26, 1938 UNITED STATES PATENT OFFICE CONVERSION OF HYDROCARBONS Application October 15, 1936,

No Drawing.

Serial N0. 105,713

4 Claims.

This invention relates particularly to the conversion of straight chain hydrocarbons into closed chain or cyclic hydrocarbons.

More specifically, it is concerned with a process involving the use of special catalysts and specific conditions of operation in regard to temperature, pressure and time of reaction whereby aliphatic hydrocarbons can be efficiently converted into aromatic hydrocarbons.

In the straight pyrolysis of pure hydrocarbons or hydrocarbon mixtures such as those encountered in fractions from petroleum or other naturally occuring or synthetically produced hydrocarbon mixtures the reactions involved which produce aromatics from paramns and olefins are of an exceedingly complicated character and cannot be very readily controlled.

It is generally recognized that, in the thermal decomposition of hydrocarbon compounds or hydrocarbon mixtures of relatively narrow range that whatever intermediate reactions are involved, there is an overall loss of hydrogen, a tendency to carbon separation and a generally wider boiling range in the total liquid products as Under mild cracking conditions involving relatively low temperatures and pressures and short times of exposure to cracking conditions it is possible to some extent to control cracking reactions so that they are limited to primary decompositions and there is a minimum loss of hydrogen and a maximum production of low boiling fractions consisting of compounds representing the fragments of the original high molecular weight compounds.

As the conditions of pyrolysis are increased in severity using higher temperatures and higher times of exposure to pyrolytic conditions, there is a progressive increase in loss of hydrogen and a large amount of secondary reactions involving recombination of primary radicals to form polymers and some cyclization to form naphthenes and aromatics, but the mechanisms involved in these cases are of so complicated a nature that very little positive information has been evolved in spite of the large amount of experimentation which has been done and the large number of theories proposed. In general, however, it may be said that, starting with parafiin hydrocarbons representing the highest degree of saturation, these compounds are changed progressively into olefins, naphthenes, aromatics, and finally into carbon and hydrogen and other light fixed gases. It is not intended to infer from this statement that any particular success has "attended the conversion of any given paraflin or other aliphatic hydrocarbon into an aromatic hydrocarbon of the same number of carbon atoms by way of the progressive steps shown. If this is done it is usually with very low yields which are of very little practical significance.

The search for catalysts to specifically control and accelerate desired conversion reactions among hydrocarbons has been attended with the usual difiiculties encountered in finding catalysts for other types of reactions since there are no basic laws or rules for predicting the effectiveness of catalytic materials and the art as a whole is in a more or less empirical state. In using catalysts even in connection with conversion reactions among pure hydrocarbons and particularly in connection with the conversion of the relatively heavy distillates and residua which are available for cracking, there is a general tendency for the decomposition reactions to proceed at a very rapid rate, necessitating the use of extremely short time factors and very accurate control of temperature and pressure to avoid too extensive decomposition. There are further difiiculties encountered in maintaining the eificiency of catalysts employed in pyrolysis since there is usually a rapid deposition of carbonaceous materials on their surfaces and in their pores.

The foregoing brief review of the art of hydrocarbon pyrolysis is given to furnish a general background for indicating the improvement in such processes which is embodied in the present invention, which may be applied to the treatment of pure paraifin' or olefin hydrocarbons, hydrocarbon mixtures containing substantial percentages of parafiin hydrocarbons such as rela- REISSUED tively close cut fractions producible by distilling of relatively low catalytic activity supporting minor proportions of oxides of elements selected from those occurring in the left-hand columns of Group IV of-the periodic table, these oxides having relatively high catalytic activity.

According to the present invention, aliphatic or straight chain hydrocarbons having 6 or more carbon atoms in chain arrangement in their structure are specifically dehydrogenated in such a way that the chain of carbon atoms undergoes ring closure with the production in the simplest case of benzene from n-hexane or n-hexene and in the case of higher molecular weight parafiins of various alkyl derivatives of benzene. Under properly controlled conditions of times of contact, temperature and pressure, very high yields of the order of '75 to 0f the benzene or aromatic compounds are obtainable which are far in excess of any previously obtained in the art either with or without catalysts. For the sake of illustrating and exemplifying the types of hydrocarbon conversion reactions which are specifically accelerated under the preferred conditions by the present types of catalysts, the following structural equations are introduced.

CH CH +411, CH:\ /GH: CH CH CH: CH

n-hexane benzene C--OH: CH:

I CH CH CH; CHr-CH; F2 +411, CH: CH; CH CH CH: CH

n-heptane toluene on 0-4311, 0]]13 CHz-CH: a HH. OH:\ CHr-CH: CH C-GH;

CH: C n-octane o-xylene In the foregoing table the structural formulas of the primary parafiin hydrocarbons have been represented as a nearly closed ring instead of by the usual linear arrangement for the sake of indicating the possible mechanisms involved. No

1 attempt has been made to indicate .the possible intermediate existence of mono-olefins, diolefins, hexamethylenes or alkylated hexamethylenes which might resultfrom the loss of various amounts of hydrogen. It is not known at the present time whether ring closure occurs at the loss of one hydrogen molecule or whether dehydrogenation of the chain carbons occurs so that the first ring compound formed is an aromatic such as benzene or one of its derivatives. The above three equations are of a relatively simple character indicating generally the type of reactions involved but in the case of n-paraffins or mono-olefins of higher molecular weight than the octane shown and in the case of branch chain compounds which contain various alkyl substituent groups in different positions along the sixcarbon atom chain, more complicated reactions will be involved. For example, in the case of such a primary compound as 2,3-dimethyl hexane the principal resultantproduct is apparently o-xylene although there are concurrently produced definite yields of such compounds as ethyl benzene indicating an isomerization of two substituent methyl groups. In the case of nonanes which are represented by the compound 2,3,4- trimethyl hexane, there is formation not only of mesitylene but also of such compounds as methyl ethyl benzol and various propyl benzols.

It will be seen from the foregoing that the scope of the present invention is preferably limited to the treatment of aliphatic hydrocarbons which contain at least 6 carbon atoms in straight .of the desired alkylated aromatics decrease in proportion.

According to the present invention composite catalytic materials are employed which comprise in general major proportions by weight of granular activated aluminum oxide as a base catalyst or supporting material for minor proportions of oxides of the elements in the lefthand column of Group IV of the periodic table comprising the elements titanium, zirconium, cerium, hafnium and thorium. The base material comprising aluminum oxide is of relatively low catalytic activity while the oxides of the elements mentioned are of relatively high catalytic activity and furnish by far the greater proportion of the observ'ed catalytic effects. The oxides of these sev-' eral elements vary somewhat in catalytic activity in any given reaction comprised within the scope of the invention and this variation may further vary in the case of different types of dehydrogenation and cyclization reactions. Some of the properties of these catalytically active oxides, which are developed on the surface and in the pores of the alumina particles will be described in succeeding paragraphs.

It should be emphasized that in the field of catalysis there have been very few rules evolved which will enable the prediction of what materials will catalyze a given reaction. Most of the catalytic work has been done on a purely empirical basis, even though at times certain groupsof elements or compounds have been found to be more or less equivalent in accelerating certain types of reactions.

Aluminum oxide which is generally preferable as a base material for the manufacture of catalysts for the process may be obtained from natural aluminum oxide minerals or ores such as bauxite or carbonates such as dawsonite by proper calcination, or it may be prepared by precipitation of aluminum hydroxide from solutions of aluminum sulfate or different alums, and dehydration of the precipitate of aluminum hydroxide by heat. Usually it is desirable and advantageous to further treat it with air or other mineral which may be used as a source of aluminum oxide.

It is best practice in the final steps of preparing aluminum oxide as a base catalyst to ignite it for some time at temperatures within the approximate range of from 800F-900 C. This probably does not correspond to complete dehydration ,of the hydroxides but apparently gives a catalytic material of good strength and porosity so that it is able to resist for a long period of time the deteriorating effects of the service and regeneration periods to which it is subjected.

My investigations have also definitely demonstrated that the catalytic efficiency of alumina, which may have some catalytic potency of itself, is greatly improved by the presence of oxides of the preferred elements in relatively minor amounts, usually of the order of less than 10% by weight of the carrier. It is most common practice to utilize catalysts comprising 2 to 5% by Weight of these oxides, particularly the lower oxides.

The oxides which constitute the principal active catalytic materials may be deposited upon the surface and in the pores of. the activated alumina granules by several altematemethods such as, for example, the ignition of nitrates which have been adsorbed or deposited from aqueous solution by evaporation or by a similar ignition of precipitated hydroxides. As an alternative method, though obviously less preferable, the finely divided oxides may be mixed mechanically with the alumina granules either in the wet or the dry condition. The point of achieving the most uniform practical distribution of the oxides on the alumina should constantly be borne in mind since the observed catalytic effects evidently depend principally upon a surface action.

The nitrate of titanium having the formula 5TiOz-N2O5--6H2O is readily soluble in moder ately warm water so that the solutions can be used as a means of initially adding a deposit of tatanium nitrate to properly prepared alumina granules. The hexahydrate may be deposited by the addition of alkali hydroxides or carbonates to thenitrate solution and in' either event the deposit is then ignited to form the dioxide T102. This compound is the principal oxide of titanium but it undergoes some reduction to the sesquialkalis are added to precipitate a tetra-hydroxide which is later ignited and dehydrogenatedto form the active dioxide. The tetrachloride and other halogen compounds are also soluble in water and may be used as solutions from which the hydroxide is precipitated. K

In the case of the element cerium the solution of the nitrate may be employed to add first the nitrate and then the cerium oxides. As before hydroxides may be precipitated which are dehydrogenated by ignition to yield oxides.

There is but one known oxide of hafnium, the

. dioxide Hi0: and this oxide is not readily reducible and probably exists as such when used in minor proportions as a constituent of catalyst composites in reactions of the present character. This oxide may be distributed mechanically upon alumina particles or developed in situ by the ignition of such compounds as the oxalate which is soluble in water in the presence of an excess of oxalic acid. Though it has catalytic activity, hafnium dioxide is seldom preferable as a catalyst on account of the rarity of the compounds of this element and their consequent cost.

The principal oxide of thorium which is used alternatively in the present instance isthe dioxide ThOz although various investigators have claimed to have proven the existence of such oxides as the pentatrioxide ThsOs, a peroxide having the. formula T1120? and a monoxide Th0. The dioxide is readily produced by the ignition of the thorium nitrate which is very soluble in water and easily used as a means of adding nitrate to aluminum granules. If desired the tetraoxide Th(OH) 4 may be precipitated from solutions of the nitrate or the halides and the, precipitate then ignited to form the dioxide. It is probable that the dioxide undergoes very little reduction during service.

, f It has been found essential to the production of high yieldsof aromatics from aliphatic hydrocarbons when using the preferred types. of catalysts that depending upon the aliphatic hydrocarbon or mixture of hydrocarbonsbeing treated, temperatures from 100-700 C. should be employed, contact times of approximately 6 to 50 seconds and pressures approximating atmospheric. The use of subatmospheric pressures of the order of A. atmosphere may be beneficial in that reduced: pressures generally favor selective dehydrogenation reactions but on the other hand moderately superatmospheric pressures usually of the order of less than 100 lbs. per sq. in. tend to increase the capacity of commercial plant equipment so that in practice a balance is struck between these two factors. most commonly employed with n-parafiinic or mono-olefinic hydrocarbons having from 6-12 carbon atoms to the molecule are of the order of 6-20 seconds. familiar with the art of hydrocarbon conversion in the presence of catalysts that the factors of temperature, pressure and time will frequently have to be adjusted from the results of preliminary experiments to produce the best results in any given instance. The criterion of the yield of aromatics will serve to fix the best'conditions of operation. In a general sense the relations between time, temperature and pressure are preferably adjusted so that rather intensive conditions are employed of sufficient severity to insure a maximum amount of the desired cyclization reactions with a minimum of undesirable side reactions. If too short times of contact are employed the conversion reactions will not proceed beyond those of simple dehydrogenation and the yields of olefinsand diolefins will predominate over those of aromatics.

While the present process is particularly applicable to the production of the corresponding aromatics from an aliphatic hydrocarbon or a mixture of aliphatic hydrocarbons, the invention may also be employed to produce aromatics from aliphatic hydrocarbon mixtures such as distillates from paraflinic or mixed base crude petroleum.'

In this case the aromatic character of the distillates will be increased and as a rule the octane number will be higher. If desired and found feasible on a basis of concentration, the aromatics It will be appreciated'by those' The times of contact .over relatively long periods of time.

treatment with reagents capable of reacting selectively with them. Another methodof aromatic concentration will involve the use of selective solvents such as liquid sulfur dioxide, alcohols, furfural, chlorex, etc.

In operating the-process. the general procedure is to vaporize hydrocarbons or mixtures of hydrocarbons and after heating the vapors to a suitable temperature within the ranges previously specified, to pass them through stationary masses of granular catalytic material in vertical cylindrical treating columns or banks of catalyst-containing tubes in parallel connection. Since the reactions are endothermic it may be necessary to apply some heat externally to maintain the best. reaction temperature. After passing through the catalytic zone the products are submitted to fractionation to recover cuts or'fractions containing the desired aromatic product with the separation of fixed'gases, unconverted hydrocarbons and heavier residual materials, which may be disposed of in any suitable manner depending upon theircomposition. The overall yield of aromatics may be increased by recycling the uncon verted straight chain hydrocarbons. to further treatment with fresh material, although this is a more or less obvious expedient and not specifically characteristic of the present invention.

. It .is an important feature of the present process that the vapors undergoing dehydrogenation should be free from all but" traces of 'water vapor since the presence of any substantial amounts of steam reduces the catalytic selectivity of the composite catalysts to a-marked degree. In view of the empirical state of the catalytic art, it is not intended to submit a complete explanation of the reasons for the deleterious influence of water vapor on the course of the present type of catalyzed reactions, but it may be suggested that the action of the steam may be to cause a partial hydration of alumina and some of the catalytic oxides due to preferential adsorption so that in eflect the hydrocarbons are prevented from reaching or being adsorbed by the catalytically active surface.

The present types of catalysts are particularly effective in removing hydrogen from chain compounds in-such a way that cyclization may be promoted without removal of hydrogen from end carbon atoms so that both end and side alkyl groupsmay appear as, substituents in benzene rings and it has been found that under proper operating conditions they do not tend to promote any great amount of undesirable'side reactions leading to thedeposition of carbon or carbonaceoils materials and for this reason show reactivity When their activity begins to diminish after a period of service, it is readily regenerated by the simple expedien't of oxidizing with air or other oxidizing gas at a moderately elevated temperature, usually within the range employed in the dehydrogenation and cyclization reactions. .This oxidation effectively removes traces of carbon deposits which contaminate the surface of the particle and decrease their emciency. It is characteristic of the present types of catalysts that they may be repeatedly regenerated with only a very gradual loss of catalytic efllciency.

During oxidation with air or other oxidizing gas mixture in regenerating partly spent, material, there is evidence to indicate that the lower oxides are to a large extent, if not completely, oxidized to higher oxides'which combine with aluminum oxide to form aluminum salts of variable composition. Later these salts are'decomposed by contact with reducing gases in the first stages of service to reform the lower. oxides and regenerate the real catalyst and hence the catalytic activity.

Example I- A catalyst was made by first dissolving titanium nitrate in cold water to make a substantially saturated solution and this was added to about 2 times its weight of activated alumina from the calcination of bauxite, the alumina particles being approximately 8-12 mesh in size. By heating in careful stages the temperature was finally brought to 350 C. andthe deposited titanium oxides were subjected to the action of a current of hydrogen at about 500 C.

-n-Hexane was now vaporized and passed over the granular catalyst particles at a temperature of 515 C., substantially atmospheric pressure, and time of contact of about 20 seconds. The

yield of benzol in a single pass was 50% by weight of the hexane charged and the ultimate yield was raised to 76% by recycling.

. Example 11 The catalyst was prepared by first precipitating zirconium hydroxide on 10-20 mesh activated alumina particles by adding a solution of sodium hydroxide to a solution of zirconium sulfate in which the alumina particles "were suspended;

Ignition of the hydroxide to produce zirconium dioxide left the desired residue of catalytic material.

In this case n-heptane was passed over the catalyst at a temperature of 550 C., atmospheric pressure and 16 seconds contact time to produce about49% weight yield of toluene this being ultimately brought to about by complete recycling of unconverted products.

. Example III This example is introduced to indicate the possibilities in manufacturing benzol from hexenes according to the present process. Using a zirconium oxide catalyst prepared generally in accordance with the. procedure outlined in Examplev II, l-hexenewas passed over the granular material at a temperature of 505 C., atmospheric pressure and about 18 seconds contact time. The yield of benzol was approximately 70% in a single pass and this could be increased to substantially by recycling of the unconverted olefin.

Example IV ultimate yield ,by complete recycling of unconverted charge.

Example V To prepare a catalyst-supporting oxides of thorium, a moderately concentrated solution of thorium nitrate was added to an. activated alumina, and on ignition a residue of about 5% by weight of thorium oxide was left. After reducing this oxide with hydrogen at a low red heat for several hours, the catalyst was placed in service.

n-Hexane was heated at a temperature of 525 C., atmospheric pressure, and 22 seconds time of contact to give a yield of 48% benzol in one pass, which was brought ultimately to 7 by complete recycling of unconverted charge. I

The foregoing specification and examples show clearly the character of the invention and the results to be expected in its application to aliphatic hydrocarbons, although neither section is intended to be unduly limiting.

I claim as my invention:

1. A process forthe production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period 01' about 6 to 50 seconds, in the presence of an aluminum oxide catalyst containing a relatively small amount of an oxide of a metal fromthe left hand column of Group IV of the periodic table and selected from the class consisting of titanium, zirconium, cerium, hafnium and thorium.

2. A process tor the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of about 6 to 50 seconds, in the presence of an aluminum oxide catalyst containing a relatively small amount of an oxide of titanium.

3. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclizlng the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of about 6 to 50 seconds, in the presence of an aluminum oxide catalyst containing a relatively small amount of an oxide of a zirconium.

4. A process for the production of aromatic hydrocarbons from aliphatic hydrocarbons of from six to twelve carbon atoms, which comprises dehydrogenating and cyclizing the aliphatic hydrocarbon by subjection to a temperature of the order of 400 to 700 C. for a period of about 6 to 50 seconds, in the presence or an aluminum oxide catalyst containing a relatively small amount of an oxide or a cerium.

JACQUE 0. MOB-BELL 

